Short circuits are a common cause of problems in newly manufactured printed circuit boards. Newly manufactured printed circuit boards often contain shorts between traces which prevent the circuits on the board from functioning properly. The difficulty of identifying and locating a short can vary with the function of the circuits or area of the circuits involved. When the short is between two traces, such as signal traces, it may go unidentified for some time. If the short occurs in a critical area of the board, such as within the connections intimate to a microprocessor found on the board it may be very difficult and may not be cost effective to identify and locate such a short.
On printed circuit boards, in the past, locating the shorts has created a great deal of difficulty. Circuits which are intimate to the microprocessor in an electrical sense may, in fact, be spread throughout the printed circuit board which may be quite large. The problem in finding the short is that the two shorted traces may be adjacent to each other at many points throughout the board. In the past, these points may be found only through tedious and error prone visual inspection of a blank board. If the board has components mounted on it, the visual inspection is made even more difficult.
Some of these shorts, which are often residual copper left on the board in manufacture, are so fine as to be virtually invisible without the aid of a low power microscope. While some shorts may actually be large blobs of copper solder, some may actually be traces placed on the board due to a revision of the board coupled with a layout error. Sometimes, the short may be located on an inner layer of the multi-layer board and therefore is not visually identifiable.
In the past, attempts have been made to by-pass the visual searching techniques by using the fact that the short completes a circuit where this circuit should not be completed at all. A relatively high current, low voltage signal is injected into the two circuits that are not supposed to be connected but are connected by means of the short. By tracing the voltage drop from one injection point to the other, the short is found at the point where the voltage drop passes from one trace to an adjacent trace. Milli-ohmmeters have been used in this regard to locate shorts. This method is subject to many problems. For example, there can be no power applied to the circuit board while it is being tested. The Milli-ohmmeter may drift causing erronous readings. Contact resistance of the probes may vary from point to point, causing misleading readings.
Another technique which has been used in the past is to use a Hall effect device in a special probe to "sniff" current flowing through the trace. This method may not work when the failure is on an inner layer of a multi-layer circuit board and also is sensitive to other electromagnetic fields which may be present.